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Unread 05-20-2014, 02:04 PM   #1
kenh
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1989 YJ Wrangler 
 
Join Date: Sep 2011
Location: Billings, MT
Posts: 97
4.2 randomly running bad

Hey all,

Have a 4.2 with a MC2150 carb I installed a few weeks and stock distributor. For the most part it's been running pretty well, but I've been having issues with a hesitation on acceleration. It didn't happen all the time though, and for a while I thought it maybe only happened once the engine got warm but I don't think that's the case.

Yesterday I decided to change the distributor to use manifold vacuum instead of ported and for the rest of the day it was great, the hesitation was 95% gone, it idled better, accelerated better, etc. I thought my issues were solved.

Today I started it up and it sounded like crap like the timing is off, idled rough, big hesitations on acceleration, and it's been doing this all day when hot and cold, after not changing anything from yesterday when it was running great.

I'm not sure what to look for at this point? ICM? Timing chain? Distributor?

Any help is appreciated, thanks.

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Unread 05-20-2014, 02:40 PM   #2
sardog211
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1989 YJ Wrangler 
 
Join Date: Jun 2012
Location: Gulf Coast, Alabma
Posts: 121
Check your timing as you stated, look at your power valve, if it's wet you have under the PV, it's bad, also spray some carb cleaner and check for vacuum leaks. How does it idle? What rpm do you have it idling at? Lastly how do the plugs look?

Have you done the nutter bypass yet?
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Unread 05-20-2014, 03:02 PM   #3
kenh
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1989 YJ Wrangler 
 
Join Date: Sep 2011
Location: Billings, MT
Posts: 97
Quote:
Originally Posted by sardog211 View Post
Check your timing as you stated, look at your power valve, if it's wet you have under the PV, it's bad, also spray some carb cleaner and check for vacuum leaks. How does it idle? What rpm do you have it idling at? Lastly how do the plugs look?

Have you done the nutter bypass yet?
Nutter has been done, I'll have to check the power valve, I'm not sure exactly how that's done though.

I don't believe there any vacuum leaks, but wouldn't hurt to spray some cleaner around. It's idling around 900 right now (was 800 with ported vacuum but I haven't adjusted it yet). It's not idling very good right now, but was great yesterday like I said. I also changed the plugs about a week ago which didn't resolve the issue.
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Unread 05-20-2014, 05:10 PM   #4
dancytron
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1990 YJ Wrangler 
 
Join Date: Jun 2008
Location: st. louis
Posts: 1,369
You probably need to adjust the vacuum advance. Manifold vacuum is a lot more powerful than ported vacuum, so you can't just switch them. You just need to stick an allen wrench into the place where the tube hooks on. Search, there are threads on how to to it and what allen wrench you need.
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Unread 05-20-2014, 05:51 PM   #5
kenh
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1989 YJ Wrangler 
 
Join Date: Sep 2011
Location: Billings, MT
Posts: 97
Quote:
Originally Posted by dancytron View Post
You probably need to adjust the vacuum advance. Manifold vacuum is a lot more powerful than ported vacuum, so you can't just switch them. You just need to stick an allen wrench into the place where the tube hooks on. Search, there are threads on how to to it and what allen wrench you need.
Thanks, I'll give that a shot. Does the base timing need to be set with the vacuum hooked up to the distributor when using manifold vacuum, since it's pulling at idle?
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Unread 05-20-2014, 07:53 PM   #6
Old4X
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Quote:
Originally Posted by kenh View Post
Thanks, I'll give that a shot. Does the base timing need to be set with the vacuum hooked up to the distributor when using manifold vacuum, since it's pulling at idle?

Set base timing with vac advance un hooked and line plugged.

Set idle speed once you plug the vac line back in.
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